First, great news, Ella Charmed has redone her fixed worlds and has them available for download all as one .world file for each world she has redone. They no longer require importing resources through SimsPE. So follow that link in the first thread if you want to try her fix(es).
Second, I too use Overwatch and added ErrorTrap as a second must have NRaas mod. When lag was being carried over into other games after play in Lunar Lakes or a world with the tree of prosperity in it, Twallan said they were both needed to keep the game running. Of course, there was a patch for that tree later; but the mod is still invaluable at keeping a game playable after a long time.
Third, after you make all the changes to a world and have it the way you want it, use SAVE AS to name it something like IPready. Then, immediatedly use SAVE AS again to call it IPplay. You can then MOVE or COPY that save file outside the Sims 3 folder as far as the EA folder to keep it safe for when you want to start a new game. I have completely redone BP with real apartment buildings versus the shells and I have redone all the buildings in Sunset Valley my own way. I can just COPY SVready into my Sims 3 folder whenever I want to start a new game.
I've been redoing all the changes in IP as I ran into a issue with a pregnant Sim trying to go to work in Politics but unable to change into a career outfit and ending up not going, but taking a hit on her work meter. I had put other mods in (Master Controller and Story Progression) because I wanted to have an elementary and a high school. So, now I need to test without those mods. But I do wonder if that is why the Collins and the Scotts have so many issues (because the moms are both pregnant).
The first go round of IP worked well for me. I took a sim diving with no problems to the first two dive spots. And everything appeared to work okay until I got a notice for Rosemarie Collins. So I went and played them for a day and found that she was scheduled to work, but couldn't. I've read this sometimes happen in Medical and Politic fields and even Science and Military or Law Enforcement (but that was directly related to one of the additional mods.)
Anyway, I am working on making a short, sweet, to the point list of just what is easiest to do yourself. BUT I do recommend trying Ella's fixes now that they are not so complicated any more. :smiley: Just backup the original and the fixed world(s) AND start a new game to test them. They can not work on a save file you've already played.